It has become increasingly common to wind long lengths of optical fibers to form wound fiber items. Typically, these optical fibers are wound onto a bobbin over a wire baselayer. The wire base layer is used to set the spacing, winding pitch, and other characteristics of the final wound optical fiber item.
A typical prior art winding system is shown in FIGS. 1A and 1B. As shown in those figures, typically a wound filament item 2 is made by first winding onto a bobbin 4 and a wire baselayer 6. The optical fiber 8 is then wound over the wire baselayer 6 until a first optical fiber layer 10 is formed. Winding of the optical fiber is then continued until a plurality of optical fiber layers are formed above the baselayer 6 and the first layer 10. The baselayer 6 and the first layer 10 abutt lips 12, 14 on bobbin 4. A close up view of one end of the bobbin 4 at lip 12 is shown in FIG. 1B.
Several problems are caused by winding the optical fibers over a wire base layer as is common in the prior art. First, we have noticed that a marked attenuation of the transmissivity of the optical fiber occurs in the first layer wound over the wire baselayer. It is our belief that such increased losses result from higher stresses, or microbends in the optical fibers in the first layer. A stylized illustration showing the relative losses in the wound fiber item 8 except for the first layer and in the first layer itself is shown in FIG. 1C. In order to appreciate the nature of this attenuation problem, it should be noted that typical wound optical fiber items for long payout applications may have up to 21 layers with 500 meters of optical fiber per layer. Commonly used optical fibers have a characteristic loss of 1/2 db per kilometer in the wound layers, excluding the first layer. However, the first layer may have up to 1 db of loss when wound over a wire baselayer at laboratory temperatures, and this first layer loss increases further at cold temperatures (e.g., at - 25.degree. F. by a factor of 10). To compensate for such attenuation, either the total fiber length must be decreased or the optical input power must be increased. In applications where total packaging and system weight and size are of concern, such increased attenuation requires a tradeoff with other system components, such as the power supply, light source (i.e., a laser), or component life.
Another disadvantage to the use of a wire base-layer is that the wire used for the baselayer is expensive. Since the baselayer wire is used to control the winding pitch and other characteristics of the final wound fiber item, the wire must be fabricated to tight tolerances, requires the use of special fabrication equipment, must be stocked, and must be inspected.
One third problem with the use of a wire base-layer is that the thermal expansion properties of the baselayer wire differs markedly from the properties of the optical fiber. Thus, there is an increased possibility that the wound fiber will slip off the metal baselayer where the wound fiber is paid out at temperatures significantly different than the winding temperature.
A fourth problem is that the winding of the basewire adds extra set to the manufacturing of the wound filament, thereby increasing cost.